Can’t fathom loss of inidicator/brake light

[MilesnMiles]

..on my Dakota truck. Lost just rear left side brake/indicator
Changed bulb
Swapped over bulb holders from left to right
Seem to have identical voltage showing when probing both left and rear side bulb holder wiring plugs.
The faulty side has side light still working

What have I missed? Surely a faulty flasher unit wouldn’t just lose one corner of one side of the vehicle?

[Mossy68]

First thing I would check mate is earths.
Add some if you need.
Was always the cause of lighting problems on my early truck.

[Rebel]

Is it possible that you have two flasher units?
Maybe one for indicator and one for hazard lights?
Sometimes things are made over complicated for the hell of it

[MilesnMiles]

The hazards don’t work on the on the rear left either. I removed the earth and cleaned it and the attached to no avail.
Current outputs appear to be identical on both sides with my multimeter. Surely I’d have no voltage to the u/s side?

[Dave999]

power on the wire to the bulb but the bulb doesn't light indicates no route to earth. it gets there....but it can't flow through to earth and back to the battery negative.

hold the wire to the connector on the bulb that sticks out the bottom with the indicator switched on

hold the shank of the bulb in some pliers with no rubber on the handles

drive the plier handles into the bumper mount or other none painted clean metal on the body so they earth and see if it lights.

if it does

run a cable from the outside of the bulb holder to a mounting screw on the light unit. make sure that mounting screw screws into metal self tapping style and not a plastic body plug.

if you can't find an earth, drill a hole in the chassis rail and screw in a nice big headed cheese head screw to do the job instead

if the lamp units are all plastic and the holder has no obvious earth route through the lamp body one of the wires is an earth wire the others are light cluster functions, and you need to find out where it goes and check it is actually earthed

or it goes back to the ECU and is earthed there to allow the ECU to decide the bulb is blown (i sent a flash Voltage to the bulb on the power lead and saw nothing on the earth lead -> bulb is out). you would see the light-out idiot light on the dash if the ECU is clever enough.

its a bit complicated
some flash the bulb by pulsing power to it, and the earth is in place at the light, all the time. no power on power lead, no power on earth lead, no 12 volt difference and the bulb, so the bulb is not lit. 12 volt on power lead, 0 volt at earth 12 volt difference and light on.

some have power to the cluster all the time the ignition is on. and an earth lead runs back to the ECU. for the light to be off the ECU applies power to the earth lead as well so the bulb sees 12 volts power on both sides of the flament = no difference in voltage between the two= no flow of power = no light
until the light needs to come on and the ECU connects the earth lead to earth via its screws to the chassis which leaves the power lead at 12 volts and the earth lead 0 volts = 12 volt difference and the light comes on.

if the power lead to your indicator bulb is 12 volts all the time and, you have no obvious earth for the bulb holder and you have another lead to the bulb that pulses when the indicator is activated. your indicators may work this second, more complicated way....but we are talking about a late 80s to late 90s truck so i'd eat my hat if it was the more complicated method. truck lights get smashed. trucks lead a hard life. i can't think they'd go with a constant live to the rear end clusters but you never know

Dave

[db]

Is the lighting still wired to U.S. spec?
Dusty is and the brake lights failed. It turned out to be a busted wire in the indicator mechanism under the steering wheel.

[MilesnMiles]

Thanks guys and Dave for the detail. It does sound like an earth issue. When it stops raining I’ll pull it apart and try the instructions above.
Someone had converted it UK spec, and then some fool has converted it back to US spec! Wiring is not tidy and there are witness marks from extra lamps front and rear.

[Mossy68]

My 85 Ram ended up with virtually all bulbs having their own earth. The holders had become so rusty , that after a good clean up there was enough room to solder an earth wire to the bulb, and run it to a good ground.
It also held the bulbs in the rusted holders better and stopped the bulbs jumping out !

[MilesnMiles]

Did that on my Roadrunner Mossy, but the Dakota is a modern Car and has plastic bulb holders. I’ve interchanged from side to side with no effect.

[MattH]

cluster functions,

Had to read that twice, after nearly spitting out my cuppa!